arXiv subjects

Stefano Scopel

Publications and source records attributed to Stefano Scopel.

At least 37 records · Page 2Linked to original sources

arXiv2020

The effective theory of nuclear scattering for a WIMP of arbitrary spin

We introduce a systematic approach to characterize the most general non-relativistic WIMP-nucleus interaction allowed by Galilean invariance for a WIMP of arbitrary spin $j_χ$ in the approximation of one-nucleon currents. Five nucleon currents arise from the nonrelativistic limit of the free nucleon Dirac bilinears. Our procedure consists in (1) organizing the WIMP currents according to the rank of the $2 j_χ+1$ irreducible operator products of up to $2 j_χ$ WIMP spin vectors, and (2) coupling each of the WIMP currents to each of the five nucleon currents. The transferred momentum $q$ appears to a power fixed by rotational invariance. For a WIMP of spin $j_χ$ we find a basis of 4+20$j_χ$ independent operators that exhaust all the possible operators that drive elastic WIMP-nucleus scattering in the approximation of one-nucleon currents. By comparing our operator basis, which is complete, to the operators already introduced in the literature we show that some of the latter for $j_χ=1$ were not independent and some were missing. We provide explicit formulas for the squared scattering amplitudes in terms of the nuclear response functions, which are available in the literature for most of the targets used in WIMP direct detection experiments.

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arXiv2019

Is a WIMP explanation of the DAMA modulation effect still viable?

We show that the weakly interacting massive particle (WIMP) scenario of proton-philic spin-dependent inelastic dark matter can still provide a viable explanation of the observed DAMA effect in compliance with the constraints from other experiments. We also show that, although the COSINE-100 collaboration has recently tested the DAMA effect using the same target material, for the time being the comparison between DAMA and COSINE-100 still depends on the particle-physics model.

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arXiv2019

A DAMA/Libra-phase2 analysis in terms of WIMP-quark and WIMP-gluon effective interactions up to dimension seven

We analyze the DAMA/Libra-phase2 modulation result using a basis of 16 effective operators describing the WIMP interaction with photons, gluons and quarks up to mass dimension seven. For each operator we fix the effective theory at the scale of 2 GeV and parametrize WIMP-quark interactions in terms of two independent couplings for up-type and down-type quarks. We discuss the connection with the non-relativistic limit of the effective theory in terms of operators invariant by Galilean transformations, and the impact of the ensuing momentum-dependent Wilson coefficients due to long-range interactions or light meson poles. Most relativistic operators yield a good fit of the DAMA modulation effect, although for parameters excluded by the constraints from XENON1T and PICO60.

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arXiv2019

On the sensitivity of present direct detection experiments to WIMP-quark and WIMP-gluon effective interactions: a systematic assessment and new model-independent approaches

Assuming for Weakly Interacting Massive Particles (WIMPs) a Maxwellian velocity distribution in the Galaxy we provide an assessment of the sensitivity of existing Dark Matter (DM) direct detection (DD) experiments to operators up to dimension 7 of the relativistic effective field theory describing dark matter interactions with quarks and gluons . In particular we focus on a systematic approach, including an extensive set of experiments and large number of couplings, both exceeding for completeness similar analyses in the literature. The relativistic effective theory requires to fix one coupling for each quark flavor, so in principle for each different combination the bounds should be recalculated starting from direct detection experimental data. To address this problem we propose an approximate model-independent procedure that allows to directly calculate the bounds for any combination of couplings in terms of model-independent limits on the Wilson coefficients of the non-relativistic theory expressed in terms of the WIMP mass and of the neutron-to-proton coupling ratio $c^n/c^p$. We test the result of the approximate procedure against that of a full calculation, and discuss its possible pitfalls and limitations. We also provide a simple interpolating interface in Python that allows to apply our method quantitatively.

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arXiv2019

COSINE-100 and DAMA/LIBRA-phase2 in WIMP effective models

Assuming a standard Maxwellian for the WIMP velocity distribution, we obtain the bounds from null WIMP search results of 59.5 days of COSINE-100 data on the DAMA/LIBRA-phase2 modulation effect within the context of the non-relativistic effective theory of WIMP-nucleus scattering. Here, we systematically assume that one of the effective operators allowed by Galilean invariance dominates in the effective Hamiltonian of a spin-1/2 dark matter (DM) particle. We find that, although DAMA/LIBRA and COSINE-100 use the same sodium-iodide target, the comparison of the two results still depends on the particle-physics model. This is mainly due to two reasons: i) the WIMP signal spectral shape; ii) the expected modulation fractions, when the upper bound on the time-averaged rate in COSINE-100 is converted into a constraint on the annual modulation component in DAMA/LIBRA. We find that the latter effect is the dominant one. For several effective operators the expected modulation fractions are larger than in the standard spin-independent or spin-dependent interaction cases. As a consequence, compatibility between the modulation effect observed in DAMA/LIBRA and the null result from COSINE-100 is still possible for several non-relativistic operators. At low WIMP masses such relatively high values of the modulation fractions arise because COSINE-100 is mainly sensitive to WIMP-sodium scattering events, due to the higher threshold compared to DAMA/LIBRA. A next COSINE analysis is expected to have a full sensitivity for the 5$σ$ region of DAMA/LIBRA.

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arXiv2019

Probing DAMA/LIBRA in the full parameter space of WIMP effective models of inelastic scattering

We discuss the compatibility of the combined annual modulation effect measured by DAMA/LIBRA-phase1 and DAMA/LIBRA-phase2 with an explanation in terms of inelastic scattering events induced by the most general Galilean-invariant effective contact interaction of a Weakly Interacting Massive Particle (WIMP) dark matter particle of spin 0, 1/2 or 1. We take into account all the possible interferences among operators by studying the intersections among the ellipsoidal surfaces of constant signal of DAMA and other experiments in the space of the coupling constants of the effective theory. In our analysis we assume a standard Maxwellian velocity distribution in the Galaxy. We find that, compared to the elastic case, inelastic scattering partially relieves but does not eliminate the existing tension between the DAMA effect and the constraints from the null results of other experiments. Such tension is very large in all the parameter space with the exception of a small region for WIMP mass $m_χ\simeq$ 10 GeV and mass splitting $δ\gtrsim$ 20 keV, where it is partially, but not completely relieved. In such region the bounds from fluorine targets are evaded in a kinematic way because the minimal WIMP incoming speed required to trigger upscatters off fluorine exceeds the maximal WIMP velocity in the Galaxy, or is very close to it. As a consequence, we also find that the residual tension between DAMA and other results is more sensitive on the astrophysical parameters compared to the elastic case. We find that the configurations with the smallest tension can produce enough yearly modulation in some of the DAMA bins in compliance with the constraints from other experiments, but the ensuing shape of the modulation spectrum is too steep compared to the measured one. For such configurations the recent COSINE-100 bound is evaded in a natural way due to their large expected modulation fractions.

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arXiv2019

Present and projected sensitivities of Dark Matter direct detection experiments to effective WIMP-nucleus couplings

Assuming for Weakly Interacting Massive Particles (WIMPs) a Maxwellian velocity distribution in the Galaxy we explore in a systematic way the relative sensitivity of an extensive set of existing and projected Dark Matter (DM) direct detection experiments to each of the 14 couplings that parameterize the most general non-relativistic (NR) effective Hamiltonian allowed by Galilean invariance for a contact interaction driving the elastic scattering off nuclei of WIMPs of spin 1/2. We perform our analysis in terms of two free parameters: the WIMP mass $m_χ$ and the ratio between the WIMP-neutron and the WIMP-proton couplings $c^n/c^p$. We include the modified signal spectral shape due to non-standard interactions when it is needed in the determination of the bound, such as in the case of background subtraction or of the application of the optimal-interval method. For each coupling, in the $m_χ$-$c^n/c^p$ plane we provide contour plots of the most stringent 90 % C.L. bound on the WIMP-nucleon cross section and show the experiment providing it. We also introduce NRDD_constraints, a simple interpolating code written in Python that allows to obtain the numerical value of the bound as a function of the WIMP mass $m_χ$ and of the coupling ratio $c^n/c^p$ for each NR coupling. We find that 9 experiments out of the 14 present Dark Matter searches considered in our analysis provide the most stringent bound on some of the effective couplings for a given choice of $(m_χ,c^n/c^p)$: this is evidence of the complementarity of different target nuclei and/or different combinations of count-rates and energy thresholds when the search of DM is extended to a wide range of possible interactions.

hep-ph↗

arXiv2019

Proton-philic spin-dependent inelastic Dark Matter (pSIDM) as a viable explanation of DAMA/LIBRA-phase2

We show that the Weakly Interacting Massive Particle scenario of proton-philic spin-dependent inelastic Dark Matter (pSIDM) can still provide a viable explanation of the observed DAMA modulation amplitude in compliance with the constraints from other experiments after the release of the DAMA/LIBRA-phase2 data and including the recent bound from COSINE-100, that uses the same $NaI$ target of DAMA. The pSIDM scenario provided a viable explanation of DAMA/LIBRA--phase1 both for a Maxwellian WIMP velocity distribution and in a halo-independent approach. At variance with DAMA/LIBRA-phase1, for which the modulation amplitudes showed an isolated maximum at low energy, the DAMA/LIBRA-phase2 spectrum is compatible to a monotonically decreasing one. Moreover, due to its lower threshold, it is sensitive to WIMP-iodine interactions at low WIMP masses. Due to the combination of these two effects pSIDM can now explain the yearly modulation observed by DAMA/LIBRA only when the WIMP velocity distribution departs from a standard Maxwellian. In this case the WIMP mass $m_χ$ and mass splitting $δ$ fall in the approximate ranges 7 GeV $\lesssim m_χ\lesssim$ 17 GeV and 18 keV$\lesssimδ\lesssim$29 keV. The recent COSINE-100 bound is naturally evaded in the pSDIM scenario due to its large expected modulation fractions.

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arXiv2018

Anapole Dark Matter after DAMA/LIBRA-phase2

We re-examine the case of anapole dark matter as an explanation for the DAMA annual modulation in light of the DAMA/LIBRA-phase2 results and improved upper limits from other DM searches. If the WIMP velocity distribution is assumed to be a Maxwellian, anapole dark matter is unable to provide an explanation of the DAMA modulation compatible with the other searches. Nevertheless, anapole dark matter provides a better fit to the DAMA-phase2 modulation data than an isoscalar spin-independent interaction, due to its magnetic coupling with sodium targets. A halo-independent analysis shows that explaining the DAMA modulation above 2 keVee in terms of anapole dark matter is basically impossible in face of the other null results, while the DAMA/LIBRA-phase2 modulation measurements below 2 keVee are marginally allowed. We conclude that in light of current measurements, anapole dark matter does not seem to be a viable explanation for the totality of the DAMA modulation.

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arXiv2018

Effective models of WIMP Direct Detection in DAMA/LIBRA-phase2

Since the DAMA/LIBRA collaboration released updated results from their search for the annual modulation signal expected from Dark Matter (DM) scattering in their NaI detectors, we have fitted the updated DAMA result for the modulation amplitudes for a Weakly Interacting Massive Particle (WIMP) signal, parameterizing the interaction with nuclei in terms of the most general effective Lagrangian for a WIMP particle spin 1/2, systematically assuming dominance of one of the 14 possible interaction terms and a standard Maxwellian for the WIMP velocity distribution. We find that most of the couplings of the non-relativistic effective Hamiltonian can provide a better fit compared to the standard Spin Independent interaction case, and with a reduced fine-tuning of the three parameters (WIMP mass, WIMP-nucleon effective cross-section and ratio between the WIMP-neutron and the WIMP-proton couplings). In addition, effective models for which the cross section depends explicitly on the WIMP incoming velocity can provide a better fit of the DAMA data at large values of $m_χ$ compared to the standard velocity-independent cross-section due to a different phase of the modulation amplitudes. All the best fit solutions are in tension with exclusion plots of both XENON1T and PICO60.

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arXiv2018

Non-standard scaling laws of WIMP-nucleus interactions in WIMP direct detection

Guided by the non-relativistic effective field theory of interactions between Weakly Interacting Massive Particles (WIMPs) of spin 1/2 and nuclei we study direct detection exclusion plots for an example of non-standard spin-dependent interaction and compare it to the standard one. We analyze an extensive list of 15 existing experiments including the effects of momentum dependence and isospin violation. In our analysis, we fixed the dark matter velocity distribution to a Maxwellian.

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arXiv2018

DAMA/LIBRA-phase2 in WIMP effective models

The DAMA/LIBRA collaboration has recently released updated results from their search for the annual modulation signal expected from Dark Matter (DM) scattering in their NaI detectors. We have fitted the updated DAMA result for the modulation amplitudes in terms of a Weakly Interacting Massive Particle (WIMP) signal, parameterizing the interaction with nuclei in terms of the most general effective Lagrangian for a WIMP particle spin up to 1/2, systematically assuming dominance of one of the 14 possible interaction terms, and assuming for the WIMP velocity distribution a standard Maxwellian. We find that most of the couplings of the non-relativistic effective Hamiltonian can provide a better fit compared to the standard Spin Independent interaction case, and with a reduced fine-tuning of the three parameters (WIMP mass, WIMP-nucleon effective cross-section and ratio between the WIMP-neutron and the WIMP-proton couplings). Moreover, effective models for which the cross section depends explicitly on the WIMP incoming velocity can provide a better fit of the DAMA data at large values of $m_χ$ compared to the standard velocity-independent cross-section due to a different phase of the modulation amplitudes. All the best fit solutions are in tension with exclusion plots of both XENON1T and PICO60.

hep-ph↗

arXiv2018

Explaining DAMA with proton-philic spin-dependent inelastic Dark Matter (pSIDM): a frequentist analysis

Proton-philic spin-dependent inelastic Dark Matter is a Weakly Inelastic Massive Particle scenario that can explain the DAMA yearly modulation effect in compliance with the constraints from other direct detection Dark Matter searches. We obtain updated ranges for its parameters both in a halo-independent approach and adopting a truncated Maxwellian for the WIMP velocity distribution constructing approximate frequentist confidence intervals from an effective chi-square including, among others, the latest experimental constraints from XENON1T, PANDAX-II, SuperCDMS and CDMSlite. In the halo-independent analysis we have implemented the dependence on the WIMP velocity distribution through a step-wise parameterization of the halo function. For the calculation of the profile likelihood we used a Markov Chain Montecarlo generator that required a large number of evaluations of the expected rates. To reduce computational time we introduced expected rates expressions suitable for a fast evaluation through tabulation and interpolation. Our frequentist analysis confirms the present viability of the pSIDM scenario as a possible explanation of the DAMA effect. In terms of the WIMP mass $m_χ$ and of the mass splitting $δ$ we find the 1-sigma ranges $\mbox{12.5 GeV} \le m_χ\le \mbox{15.7 GeV}$, $\mbox{22.1 keV} \le δ\le \mbox{26.1 keV}$ for the halo--independent analysis and $\mbox{11.4 GeV} \le m_χ\le \mbox{13.6 GeV}$, $\mbox{24.4 keV} \le δ\le \mbox{27.0 keV}$ for the Maxwellian case. We find that a full year of data taking of XENON1T should allow to start probing the pSIDM scenario.

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arXiv2017

Halo-independent determination of the unmodulated WIMP signal in DAMA: the isotropic case

We present a halo-independent determination of the unmodulated signal corresponding to the DAMA modulation if interpreted as due to dark matter weakly interacting massive particles (WIMPs). First we show how a modulated signal gives information on the WIMP velocity distribution function in the Galactic rest frame, from which the unmodulated signal descends. Then we perform a mathematically-sound profile likelihood analysis in which we profile the likelihood over a continuum of nuisance parameters (namely, the WIMP velocity distribution). As a first application of the method, which is very general and valid for any class of velocity distributions, we restrict the analysis to velocity distributions that are isotropic in the Galactic frame. In this way we obtain halo-independent maximum-likelihood estimates and confidence intervals for the DAMA unmodulated signal. We find that the estimated unmodulated signal is in line with expectations for a WIMP-induced modulation and is compatible with the DAMA background+signal rate. Specifically, for the isotropic case we find that the modulated amplitude ranges between a few percent and about 25% of the unmodulated amplitude, depending on the WIMP mass.

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arXiv2017

From direct detection to relic abundance: the case of proton-philic spin-dependent inelastic Dark Matter

We discuss strategies to make inferences on the thermal relic abundance of a Weakly Interacting Massive Particle (WIMP) when the same effective dimension-six operator that explains an experimental excess in direct detection is assumed to drive decoupling at freeze-out, and apply them to the proton-philic Spin-dependent Inelastic Dark Matter (pSIDM) scenario, a phenomenological set-up containing two states $χ_1$ and $χ_2$ with $m_{χ_2}>m_{χ_1}$ that we have shown in a previous paper to explain the DAMA effect in compliance with the constraints from other detectors. We update experimental constraints on pSIDM, extend the analysis to the most general spin-dependent momentum-dependent interactions allowed by non-relativistic Effective Field Theory (EFT), and consider for the WIMP velocity distribution in our Galaxy both a halo-independent approach and a standard Maxwellian. The problem of calculating the relic abundance by using direct detection data to fix the model parameters is affected by a strong sensitivity on $f(v)$ and by the degeneracy between the WIMP local density and the WIMP-nucleon scattering cross section. As a consequence, a DM direct detection experiment is not directly sensitive to the physical cut-off scale of the EFT, but on some dimensional combination that does not depend on the actual value of the relic abundance. However, such degeneracy can be used to develop a consistency test on the possibility that the WIMP is a thermal relic in the first place. When we apply it to the pSIDM scenario we find that only a WIMP with a standard spin-dependent interaction ${\cal O}_{spin}$ with quarks can be a thermal relic, for a galactic velocity distribution that departs from a Maxwellian. However all the $χ_2$ states must have already decayed today, and this requires some additional mechanism besides that provided by the ${\cal O}_{spin}$ operator.

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arXiv2016

Inelastic dark matter with spin-dependent couplings to protons and large modulation fractions in DAMA

We discuss a scenario where the DAMA modulation effect is explained by a Weakly Interacting Massive Particle (WIMP) which upscatters inelastically to a heavier state and predominantly couples to the spin of protons. In this scenario constraints from xenon and germanium targets are evaded dynamically, due to the suppression of the WIMP coupling to neutrons, while those from fluorine targets are evaded kinematically, because the minimal WIMP incoming speed required to trigger upscatters off fluorine exceeds the maximal WIMP velocity in the Galaxy, or is very close to it. In this scenario WIMP scatterings off sodium are usually sensitive to the large-speed tail of the WIMP velocity distribution and modulated fractions of the signal close to unity arise in a natural way. On the other hand, a halo-independent analysis with more conservative assumptions about the WIMP velocity distribution allows to extend the viable parameter space to configurations where large modulated fractions are not strictly necessary. We discuss large modulated fractions in the Maxwellian case showing that they imply a departure from the usual cosine time dependence of the expected signal in DAMA. However we explicitly show that the DAMA data is not sensitive to this distortion, both in time and frequency space, even in the extreme case of a 100 % modulated fraction. Moreover the same scenario provides an explanation of the maximum in the energy spectrum of the modulation amplitude detected by DAMA in terms of WIMPs whose minimal incoming speed matches the kinematic threshold for inelastic upscatters. For the elastic case the detection of such maximum suggests an inversion of the modulation phase below the present DAMA energy threshold, while this is not expected for inelastic scattering. This may allow to discriminate between the two scenarios in a future low-threshold analysis of the DAMA data.

astro-ph.CO↗

arXiv2015

New approaches in the analysis of Dark Matter direct detection data: scratching below the surface of the most general WIMP parameter space

We show that compatibility between the DAMA modulation result (as well as less statistically significant excesses such as the CDMS Silicon effect and the excess claimed by CRESST) with constraints from other experiments can be achieved by extending the analysis of direct detection data beyond the standard elastic scattering of a WIMP off nuclei with a spin--dependent or a spin--independent cross section and with a velocity distribution as predicted by the Isothermal Sphere model. To do so we discuss several new approaches for the analysis of Dark Matter direct detection data, with the goal to remove or reduce its dependence on specific theoretical assumptions, and to extend its scope: the factorization approach of astrophysics uncertainties, the classification and study of WIMP-nucleon interactions within non--relativistic field theory, inelastic scattering and isovector-coupling cancellations including subdominant two-nucleon NLO effects. Typically, combining two or more of these ingredients can lead to conclusions which are very different to what usually claimed in the literature. This shows that we are only starting now to scratch the surface of the most general WIMP direct detection parameter space.

astro-ph.CO↗

arXiv2015

Generalized spin-dependent WIMP-nucleus interactions and the DAMA modulation effect

Guided by non-relativistic Effective Field Theory (EFT) we classify the most general spin-dependent interactions between a fermionic Weakly Interacting Massive Particle (WIMP) and nuclei, and within this class of models we discuss the viability of an interpretation of the DAMA modulation result in terms of a signal from WIMP elastic scatterings using a halo-independent approach. We find that, although several relativistic EFT's can lead to a spin-dependent cross section, in some cases with an explicit, non-negligible dependence on the WIMP incoming velocity, three main scenarios can be singled out in the non-relativistic limit which approximately encompass them all, and that only differ by their dependence on the transferred momentum. For two of them compatibility between DAMA and other constraints is possible for a WIMP mass below 30 GeV, but only for a WIMP velocity distribution in the halo of our Galaxy which departs from a Maxwellian. This is achieved by combining a suppression of the WIMP effective coupling to neutrons (to evade constraints from xenon and germanium detectors) to an explicit quadratic or quartic dependence of the cross section on the transferred momentum (that leads to a relative enhancement of the expected rate off sodium in DAMA compared to that off fluorine in droplet detectors and bubble chambers). For larger WIMP masses the same scenarios are excluded by scatterings off iodine in COUPP.

astro-ph.CO↗